The Mg-carbonate-Fe interaction:Implication for the fate of subducted carbonates and formation of diamond in the lower mantle

The fate of subducted carbonates in the lower mantle and at the core-mantle boundary was modelled via experiments in the MgCO_3-Fe~0 system at 70-150 GPa and 800-2600 Kin a laser-heated diamond anvil cell.Using in situ synchrotro n X-ray diffraction and ex situ transmission electron microscopy we show that the reduction of Mg-carbonate can be exemplified by:6 MgCO_3 + 19 Fe= 8 FeO+10(Mg_(0.6)Fe_(0.4))O+ Fe_7 C_3 + 3 C.The presented results suggest that the interaction of carbonates with Fe~0 or Fe~0-bearing rocks can produce Fe-carbide and diamond,which can accumulate in the D" region,depending on its carbon to Fe ratio.Due to the sluggish kinetics of the transformation,diamond can remain metastable at the core-mantle boundary(CMB)unless it is in a direct contact with Fe-metal.In addition,it can be remobilized by redox melting accompanying the generation of mantle plumes.